Electrochemical desalination system

ABSTRACT

A system comprises an electrodialysis apparatus, which includes first and second reservoirs, wherein a salt concentration in the first reservoir reduces below a threshold concentration, and salt concentration in the second reservoir increases during an operation mode. A first electrode comprises a first solution of a first redox-active electrolyte material, and a second electrode comprises a second solution of a second redox-active electrolyte material. In a first reversible redox reaction between the first electrode and first electrolyte material at least one ion is accepted from the first reservoir, and in a second reversible redox reaction between the second electrode and second electrolyte material at least one ion is driven into the second reservoir. A first type of ion exchange membrane is disposed between the first and second reservoirs, and a second type of ion exchange membrane, different from the first type, is disposed between the respective electrodes and reservoirs.

RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patentapplication Ser. No. 62/627,449, filed on Feb. 7, 2018, and U.S.provisional patent application Ser. No. 62/628,573, filed on Feb. 9,2018, to which priority is claimed pursuant to 35 U.S.C. § 119(e), andwhich are incorporated herein by reference in their entirety.

TECHNICAL FIELD

This disclosure relates generally to desalination-salination systemsthat are optionally capable of energy storage, methods of operating thesame, and an electrochemical battery for use in the systems.

BACKGROUND

Deployment of grid-scale electrical energy storage enables deeppenetration of energy generation from intermittently availablerenewables. Today's batteries provide the capability for load shiftingbut high prices are still holding back widespread integration ofstorage, and consequently may slow the adoption of renewables. At thesame time, rising water scarcity has forced the installation ofenergy-intensive desalination technologies to meet the growing waterdemand.

For example, there is an ever-increasing pressure on supplies of freshwater as a result of climate change and the relentless pace ofpopulation growth worldwide. For communities located in areas wherethere is no ready access to fresh water, such as the Persian Gulf andother desert areas, fresh water is produced through desalination ofseawater. In these locations, this process is highly energy intensivewhether it is driven hydraulically (e.g., through reverse osmosis (RO)),thermally (e.g., through flash distillation), or electrochemically(e.g., through electrodialysis). Elsewhere, all of these methods areroutinely employed to treat contaminated wastewater from industrialactivity.

In addition, the price of electricity generation from renewable sourcesis rapidly falling, driven primarily by technological improvements insolar and wind generation. As recently as October 2017, Saudi Arabiareceived a bid to provide electricity from solar power at a price of$17.90/MWh for a 300MW plant. This ready availability of cheap electronspresents an opportunity for electrochemical methods of waterdesalination (or treatment) to play a greater role in meeting the risingdemand for water. Described herein are systems and processes that reduceboth energy consumption and overall costs for desalination using anelectrochemical battery.

SUMMARY

Embodiments described herein are directed to a system for separatingsolvent from a salt dissolved in the solvent. The system comprises anelectrodialysis apparatus comprising first and second reservoirs, eachcomprising an input and an output, wherein salt dissolved in the solventin the first reservoir is reduced below a threshold concentration duringan operation mode and salt dissolved in the solvent in the secondreservoir increases in concentration during the operation mode. A firstelectrode comprises a first solution of a first redox-active electrolytematerial and is configured to have a reversible redox reaction with thefirst redox-active electrolyte material, and to accept at least one ionfrom the solvent in the first reservoir. A second electrode comprises asecond solution of a second redox-active electrolyte material and isconfigured to have a reversible redox reaction with the secondredox-active electrolyte material, and to drive at least one ion intothe solvent in the second reservoir. The apparatus further comprises afirst type of ion exchange membrane disposed between the first andsecond reservoirs and a second type of ion exchange membrane, differentfrom the first type. The second type of membrane is disposed between thefirst electrode and the first reservoir and disposed between the secondelectrode and the second reservoir.

Further embodiments are directed to a desalination and electricalstorage system. The system comprises an input water source comprisingwater of a first salinity, a first electrodialytic battery unit, and aswitching unit. The electrodialytic battery unit comprises a first waterreservoir coupled to the input water source and comprising an output,wherein water in the first water reservoir is reduced in salinity to asecond salinity or increased in salinity to a third salinity during anoperation mode. A second water reservoir is also coupled to the inputwater source and comprises an output, wherein salinity of the water inthe second water reservoir is changed in the opposite manner as thewater in the first water reservoir during the operation mode. A firstelectrode comprises a first solution of a first redox-active electrolytematerial and is configured to have a reversible redox reaction with thefirst redox-active electrolyte material, and to accept at least one ionfrom the water in the first water reservoir. A second electrodecomprises a second solution of a second redox-active electrolytematerial and is configured to have a reversible redox reaction with thesecond redox-active electrolyte material, and to drive at least one ioninto the water in the second water reservoir. The electrodialyticbattery further includes a first type of exchange membrane disposedbetween the first and second water reservoirs and a second type ofexchange membrane, different from the first type, disposed between thefirst electrode and the first water reservoir and disposed between thesecond electrode and the second water reservoir. The switching unitcomprises a first switch coupled to the output of the first waterreservoir and a second switch coupled to the output of the second waterreservoir.

Additional embodiments are directed to methods for separating solventfrom a salt dissolved in the solvent. The methods include providing anelectrochemical battery unit having at least first and second reservoirsand transporting solvent having a first concentration of dissolved saltinto the first and second reservoirs. The battery unit is operated in afirst mode to generate a first stream having a second concentration ofdissolved salt that is lower than the first concentration and togenerate a second stream having a third concentration of dissolved saltthat is higher than the first concentration. The electrochemical batteryunit comprises first and second reservoirs, each comprising an input andan output, wherein the salt dissolved in the solvent in the firstreservoir is reduced below a threshold concentration during an operationmode and the salt dissolved in the solvent in the second reservoirincreases in concentration during the operation mode. A first electrodecomprises a first solution of a first redox-active electrolyte materialand is configured to have a reversible redox reaction with the firstredox-active electrolyte material, and to accept at least one ion fromthe solvent in the first reservoir. A second electrode comprises asecond solution of a second redox-active electrolyte material and isconfigured to have a reversible redox reaction with the secondredox-active electrolyte material, and to drive at least one ion intothe solvent in the second reservoir. The battery unit further includes afirst type of exchange membrane disposed between the first and secondwater reservoirs and a second type of exchange membrane, different fromthe first type. The second type of membrane is disposed between thefirst electrode and the first reservoir and disposed between the secondelectrode and the second reservoir.

The above summary is not intended to describe each disclosed embodimentor every implementation of the present disclosure. The figures and thedetailed description below more particularly exemplify illustrativeembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The discussion below refers to the following figures, wherein the samereference number may be used to identify the similar/same component inmultiple figures. However, the use of a number to refer to a componentin a given figure is not intended to limit the component in anotherfigure labeled with the same number. The figures are not necessarily toscale.

FIG. 1A is a schematic diagram of an electrodialysis system with apositively charged redox shuttle in accordance with certain embodiments;

FIG. 1B is a schematic diagram of an electrodialysis system with anegatively charged redox shuttle in accordance with certain embodiments;

FIG. 2A is a schematic diagram of a charge cycle of an electrodialyticbattery with a pair of positively charged reactants in accordance withcertain embodiments;

FIG. 2B is a schematic diagram of a discharge cycle of theelectrodialytic battery of FIG. 2A in accordance with certainembodiments;

FIGS. 3A-B are a schematic diagram of an energy storage system ascoupled to a power supply unit and a power grid in accordance withcertain embodiments;

FIG. 4A is a graph of the open-circuit potential as a function of acell's state of charge in accordance with certain embodiments;

FIG. 4B is a graph of the respective reservoir salinities as a functionof a cell's state of charge in accordance with certain embodiments;

FIG. 5A illustrates the discharge operating mode for a four chamberedelectrodialytic battery in accordance with certain embodiments;

FIG. 5B illustrates the charge operating mode for a four chamberedelectrodialytic battery in accordance with certain embodiments;

FIG. 5C illustrates an electrodialysis operating mode for a fourchambered electrodialytic battery in accordance with certainembodiments;

FIG. 5D illustrates a dehumidification operating mode for afour-chambered electrodialytic battery in accordance with certainembodiments;

FIGS. 6A-B are graphs showing performance metrics of a four-chamberedelectrodialysis system utilizing ferrocyanide/ferricyanide as a redoxshuttle;

FIG. 6C is a graph showing salt concentration as a function of chargepassed in the four-chambered electrodialysis system of FIGS. 6A-B;

FIG. 6D is a graph showing salt concentration as a function of time inthe four-chambered electrodialysis system of FIGS. 6A-B;

FIG. 7A is a graph comparing operating potential and specific energyconsumption for conventional and redox-assisted electrodialysis systemsin the cell stack of FIG. 5;

FIG. 7B is a graph comparing relative specific energy consumption forconventional and redox-assisted electrodialysis systems for varyingnumber of pairs of salinate/desalinate chambers per anode and cathode;

FIGS. 8A-D are graphs of system costs, revenues, and performance metricsfor a four-chambered electrodialytic battery using conservative pricingfor water and electricity;

FIGS. 9A-D are graphs of system costs, revenues, and performance metricsfor a four chambered electrodialytic battery using optimistic pricingfor water and electricity in the Persian Gulf region;

FIG. 10 is a graph of component costs for a four-chambered cell inaccordance with certain embodiments;

FIGS. 11-13 are flow diagrams of methods in accordance with certainembodiments;

FIGS. 14A-B are graphs showing performance metrics of a four-chamberedelectrodialysis system utilizing BTMAP-Fc as a redox shuttle;

FIG. 14C is a graph showing salt concentration as a function of chargepassed in the four-chambered electrodialysis system of FIGS. 14A-B; and

FIG. 14D is a graph showing salt concentration as a function of time inthe four-chambered electrodialysis system of FIGS. 14A-B.

DETAILED DESCRIPTION

The present disclosure is generally related to electrochemicaldesalination systems and optionally, corresponding, simultaneous energystorage. Current research efforts in grid storage have been beholden toa singular approach—minimizing the cost per kWh, which currently has a

Department of Energy cost target of $125/kWh by 2022. An alternativestrategy is to increase the revenue associated with each kWh of storage,an alternative that is not available to conventional energy storagetechnologies. However, since the electrochemical batteries describedherein produce a valuable secondary product, desalinated water, duringcharging and discharging, this alternative is possible. Because aqueousflow batteries share many capital requirements (e.g. pumps, membranes,plumbing) with electrochemical desalination technologies, a system thatcombines the two could lead to significant capital cost savings comparedto two separate systems. Revenue from the desalinated water compensatesfor additional capital costs and leverages process intensification tobreak below the $/kWh barrier encountered by traditional energy storagetechnologies. Electrochemical approaches to desalination have thepotential to scale modularly and ramp production easily, whilemaintaining high energetic efficiency and the ability to processhigh-salinity feeds.

The current state of the art in electrochemical water desalination iselectrodialysis; however, it currently consumes comparatively moreenergy for salt removal (e.g., ˜0.26-0.30 kWh/kg NaCl) than otherdesalination techniques like reverse osmosis (e.g., 0.06-0.08 kWh/kgNaCl) but less than for thermal techniques like vapor compression (e.g.,0.6-1.0 kWh/kg NaCl). Capacitive deionization uses electrical energy butis also energy intensive at about 0.22 kWh/kg NaCl and is best suitedfor removing minute amounts of dissolved salts from water because theelectrodes have to be solid, by definition. While electrodialysis is atechnique that can be employed to treat brines at any salinity, unlikereverse osmosis, it has seen limited use because of its high specificenergy consumption for salt removal.

Because the energy consumption in electrodialysis is proportional to theapplied voltage, reducing (or minimizing) the voltage that has to beapplied to a cell will reduce the specific energy consumption of theelectrodialysis stack. In conventional electrodialysis, ions are drivenout of, or into, seawater by Faradaic reactions at an anode and cathode.In most cases, the Faradaic reactions are simply that of watersplitting: water is oxidized to oxygen at the anode and reduced tohydrogen at the cathode. This creates a charge imbalance at theelectrodes that is balanced by the movement of ions throughstrategically placed ion-selective membranes. However, water splittinginvolves an energetic penalty because energy is required to do so. Theproblem is exacerbated by the fact that significant overpotentials areassociated with both water oxidation and reduction. Moreover, oxygen andchlorine gas generated at the anode are highly destructive and requirethe use of platinum/iridium-plated electrodes.

In the state of California alone, if all in-state fossil fuel sources ofelectrical generation (0.40 quads, 41% of all sources) were to bereplaced by solar and paired with the desalination battery for storageas described herein, it would enable an additional 0.63 quads of solargeneration to be brought online, based on a cost-optimized round-tripenergy efficiency of 64%. At the same time, the desalination batterywould provide a water resource that is equivalent to 30% of stateconsumption.

An electrochemical cell, as described herein, is designed to performelectrodialysis in an energy-efficient manner by circulating aredox-active species that is dissolved in water from the anode to thecathode and back again. The redox-active species has rapid kinetics forreduction or oxidation, which greatly reduces the high operating voltagerequired for conventional electrodialysis, in which water splittingdrives salt transport across membranes such as ion-selective membranes.Reducing the operating voltage reduces the specific consumption ofenergy because the specific energy consumption is proportional to theoperating voltage. The system can furthermore be reversibly adapted forcoupled electrical energy storage, by pairing two separate redox couplesat the anode and cathode instead of a single redox shuttle that iscirculated around both.

In certain embodiments of an energy-efficient, low-potentialelectrodialysis system, a redox carrier that is dissolved in water isreduced at the cathode, then shuttled to the anode where it isreoxidized and subsequently redelivered to the cathode to complete thecycle. Turning to FIGS. 1A-B, an electrochemical cell 100 providingenergy-efficient electrodialysis, according to such embodiments, isshown. The cell 100 consists of four chambers 102, 104, 106, 108 inseries. Each chamber is separated from its neighbor by an appropriatemembrane 110, 112, 114 (FIG. 1A) or 116, 118, 120 (FIG. 1B). The twocentral chambers 104, 106 contain a salinate stream 130 and a desalinatestream 132, and the two outer chambers 102, 108 respectively contain thecathode and the anode (FIG. 1A) or the anode and the cathode (FIG. 1B).The membranes may be ion-selective membranes such as cation exchangemembranes or anion exchange membranes depending upon the cell design. Ifthe redox shuttles have a high enough molecular weight (e.g., aredendrimeric or polymeric in nature), the membranes may be microporousmembranes. The membranes may also incorporate some ion-selectiveelements and some microporous elements within the same membrane. Incertain embodiments, the membranes may also be composite membranes.

For example, FIG. 1A illustrates a redox-assisted electrodialysis systemwith a positively charged redox shuttle (i.e.,1,1′-bis(trimethylammoniopropyl)ferrocene dichloride (BTMAP-Fc)).Movement of the redox shuttle from the anode 108 to the cathode 102 isshown by arrow 128 and from the cathode 102 to the anode 108 by arrow126. The cathode chamber 102 and the salinate chamber 104 are separatedby an anion exchange membrane 110, and the anode chamber 108 and thedesalinate chamber 106 are also separated by an anion exchange membrane114. However, membranes 110 and 114 may not necessarily comprise thesame material or be of similar dimensions. The salinate chamber 104 isalso separated from the desalinate chamber 106 by a cation exchangemembrane 112. As can be seen, chloride and sodium ions cross membranes110 and 112 to enter the salinate stream 130 in chamber 104 while theycross membranes 114 and 112 to leave the desalinate stream 132 inchamber 106.

Using a negatively charged redox shuttle (e.g.,ferrocyanide/ferricyanide (Fe(CN))) alters the cell 100 design as shownin FIG. 1B. Here, movement of the redox shuttle from the cathode 108 tothe anode 102 is shown by arrow 128 and from the anode 102 to thecathode 108 by arrow 126. The anode chamber 102 and the salinate chamber104 are separated by a cation exchange membrane 120, and the cathodechamber 108 and the desalinate chamber 106 are also separated by acation exchange membrane 116. However, membranes 120 and 116 may notnecessarily comprise the same material or be of similar dimensions. Thesalinate chamber 104 is also separated from the desalinate chamber 106by an anion exchange membrane 118. As can be seen, chloride and sodiumions cross membranes 118 and 120 to enter the salinate stream 130 inchamber 104 while they cross membranes 116 and 118 to leave thedesalinate stream 132 in chamber 106.

In various embodiments, an arbitrary number of pairs of alternatingsalinate and desalinate chambers can be employed. However, watersplitting may start to occur at a high number of chambers once theapplied voltage exceeds 1.26 V. While solid redox carriers may beemployed in various embodiments, they require large amounts of thecarriers and frequent switching of salinate and desalinate streamsbecause solid redox-active materials cannot be easily transported fromone side of the electrochemical cell to the other.

Energy efficiency of the cell 100 is achieved through selection of theredox carrier/shuttle. An effective redox carrier possesses as many ofthe following properties as possible. For example, the redox carriershould be chemically stable in oxidized and reduced forms, remain highlywater-soluble in oxidized and reduced forms, and not be oxygen sensitivein oxidized and reduced forms. The carrier should not be proton coupled,should possess rapid redox kinetics, and should be chemically compatiblewith any component present in the water being treated. Further, thecarrier should have low permeability through ion-selective membranes andbe nontoxic.

The most popular redox carriers that have been reported to date areiron-containing compounds, including Fe²⁺/Fe³⁺, theferrocyanide/ferricyanide (Fe(CN)) couple, and Fe(II)-EDTA/Fe(III)-EDTA.All three have been considered for use in reverse electrodialysis, i.e.,the generation of electricity from salinity gradients instead of theconsumption of electricity to create a salinity gradient. The use of aredox couple for conventional electrodialysis has not yet been reported.Unfortunately, Fe³⁺ ions are only soluble at low pH and form insolubleoxides or hydroxides at neutral pH, Fe(CN) forms highly insolublePrussian Blue-type compounds upon contact with many transition metals(especially iron), and Fe-EDTA complexes show limited electrochemicalstability.

While a system with Fe(CN) as the redox carrier may be successfullydemonstrated in the laboratory with only NaCl as simulated seawater, theubiquitous presence of calcium and iron in seawater (typically at levelsof 400 parts per million (ppm) and 1-3 parts per billion (ppb),respectively) will quickly cause membrane fouling when these ions crossover into the reservoirs. There, they form insoluble precipitates ofpotassium calcium and iron upon contact with Fe(CN). Furthermore, evenneutral to slightly acidic conditions, which are created at the anode,will cause the release of highly toxic hydrogen cyanide.

However, a ferrocene derivative that possesses all of the attributeslisted above can be a suitable redox carrier, for example, BTMAP-Fc.This is compared with various iron-containing redox couples in Table 1below.

TABLE 1 Fe²⁺/Fe³⁺ Fe(EDTA) Fe(CN) BTMAP-Fc Chemical Excellent PoorExcellent (but Excellent and electro- ferricyanide is chemical lightsensitive) stability Solubility Fe³⁺ is 0.4M 0.6M 1.9M at pH 7 insolubleOxygen No No No No sensitive Kinetic rate 1.2 × 10⁻⁴ 2.6 × 10⁻² ~1 ×10⁻¹ 1.4 × 10⁻² constant on cm/s cm/s^(†) cm/s cm/s glassy carbon FormsNo No Yes No insoluble precipitates with other ions Permeability UnknownUnknown <1 × 10⁻¹¹ 6.2 × 10⁻¹⁰ cm²/s cm²/s (Nafion 212)^(‡) (SelemionDSV) Toxicity Low Low Generally low, Expected to but releases be low HCNat pH ≤7 ^(†)On platinum ^(‡)At pH 14

Notably, each of the redox couples, other than BTMAP-Fc, has at leastone property that is inconsistent with the properties of an effectiveredox carrier for the above-discussed cell. For example, Fe²⁺/Fe³⁺ isinsoluble at pH 7, Fe(EDTA) has poor chemical and electrochemicalstability, and Fe(CN) forms insoluble precipitates with other ions andreleases toxic HCN at a pH of 7 or less. In principle, any water-solubleredox carrier could be used in embodiments of the cell of FIGS. 1A-B,not just those listed in Table 1. For example, depending on the desiredpH of the desalinate and salinate streams, other redox couples may bepreferred. An example of such an energy-efficient redox assistedelectrodialysis system is described further below.

The four-chambered cell design discussed above in connection with FIGS.1A-B can also be adapted for use as an energy storage device (i.e., anelectrodialytic battery). When the cell uses a small number ofsalinate/desalinate chamber pairs (e.g., one pair), the performance isimproved over traditional electrodialysis as shown in FIG. 7B. A fewernumber of chamber pairs is also advantageous for energy storageapplications where preferred operating current density is also lowerthan that for electrodialysis. The above-described cell is adapted forenergy storage by using two different redox-active reactants as separateanolytes and catholytes instead of shuttling the same compound betweenthe anode and cathode. Unlike three-chambered cell designs (e.g., U.S.Pat. Nos. 9,340,436; 9,670,077; and 9,673,472, each of which isincorporated herein by reference), a four-chambered design is capable ofcontinuous production of desalinated water at all times duringoperation—instead of only half of the time. Also, the four-chambereddesign does not suffer from precipitation of insoluble solids ifcrossover takes place.

Embodiments of the four-chambered electrodialytic battery are shown inFIGS. 2A-B. The desalination battery is a multi-chambered flow battery.Reduction of the anolyte and oxidation of the catholyte during a chargehalf-cycle moves Na⁺ and Cl⁻ ions through appropriate ion-selectivemembranes and into, or out of, intervening chambers that hold seawater.The reverse process takes place during the discharge half-cycle. At allpoints during cycling, one of the water chambers experiences a netinflux of salt while the other sees a net efflux. The energy required toeffect the desalination is simply the difference in energy input duringcharging and the energy recovered during discharging.

In FIG. 2A, the charge cycle of a four-chambered battery using apositively charged pair of reactants is illustrated. The batteryincludes four chambers 202, 204, 206, and 208 as well as three membranes210, 212, and 214. During the charge cycle, the salinate stream 230 isin chamber 204 between the anolyte chamber 202 and chamber 206, whichcontains the desalinate stream 232. The catholyte chamber 208 isseparated from chamber 206 and the desalinate stream 232 by an anionexchange membrane 214 while the anolyte chamber 202 is separated fromchamber 204 and the salinate stream 230 by another anion exchangemembrane 210. As discussed above, membranes 210 and 214 may notnecessarily comprise the same material or be of similar dimensions. Thesalinate chamber 204 is also separated from the desalinate chamber 206by a cation exchange membrane 212. During the charge cycle, chloride andsodium ions cross membranes 210 and 212 to enter chamber 204 forming thesalinate stream 230 while they cross membranes 214 and 212 to leavechamber 206 forming the desalinate stream 232. FIG. 2B shows the samebattery of FIG. 2A during a discharge cycle. Thus, chloride and sodiumions are shown crossing membranes 210 and 212 to leave chamber 204forming the desalinate stream 232 while they cross membranes 214 and 212to enter chamber 206 forming the salinate stream 230. Notably, if thereactants were negatively charged, the membranes would be reversed:membranes 210, 214 would be cation exchange membranes and membrane 212would be an anion exchange membrane. As before, membranes 210 and 214may not necessarily comprise the same material or be of similardimensions when they are cation exchange membranes. In the embodimentsshown, the anolyte is zinc and the catholyte is BTMAP-Fc. However, inother embodiments, (ferrocenylmethyl)trimethylammonium chloride (FcNCl)can be used in place of BTMAP-Fc to increase the battery cell potentialto about 1.41 V.

The anolytes and catholytes are not restricted to the above-describedembodiments. The redox-active component of the anolyte and/or catholytecan be an aqueous solution of any combination of the following, in oneor more of their oxidation states, as their ions or oxocations oroxoanions and/or complexed to ligand(s): titanium(III), titanium(IV),vanadium(II), vanadium(III), vanadium(IV), vanadium(V), chromium(II),chromium(III), chromium(VI), manganese(II), manganese(III),manganese(VI), manganese(VII), iron(II), iron(III), iron (VI),cobalt(II), cobalt(III), nickel(II), copper(I), copper(II), zinc(II),ruthenium(II), ruthenium(III), tin(II), tin(IV), cerium(III),cerium(IV), tungsten(IV), tungsten(V), osmium(II), osmium(III),lead(II), zincate, aluminate, chlorine, chloride, bromine, bromide,tribromide, iodine, iodide, triiodide, polyhalide, halide oxyanion,sulfide, polysulfide, sulfur oxyanion, ferrocyanide, ferricyanide, aquinone derivative, an alloxazine derivative, a flavin derivative, aviologen derivative, a ferrocene derivative, any other metallocenederivative, a nitroxide radical derivative, a N,N-dialkyl-N-oxoammoniumderivative, a nitronyl nitroxide radical derivative, and/or polymersincorporating complexed or covalently bound components of any of theaforementioned species.

The anolyte and catholyte may also include an aqueous solution of thecomponents of a pH buffer that may or may not be redox-active undertypical operating conditions. In certain aqueous embodiments, the pH ofthe anolyte and catholyte is matched to the pH of the electrolyte in thecentral chambers, which may, for example, be near neutral (pH 5-9) forwater desalination, acidic (pH 0-5) for treating acidic wastewater, oralkaline (pH 9-14) for treating alkaline wastewater. In someembodiments, it can be advantageous for the anolyte pH to be slightlylower than the other chambers such as when the anolyte is zinc/zincchloride. In further embodiments, the pH of each of the electrolytes inthe system is substantially the same within the electrochemical cell. Instill further embodiments, the anolyte, catholyte, and water each has apH between and including 3-10. Thus, the cell may include a pHmonitoring and adjustment system for periodic and/or continuous pHmonitoring and adjustment.

In further embodiments, an electryodialytic battery cell as described inFIGS. 2A-B can be coupled to an energy storage system when designed as aflow battery. Flow batteries are attractive for energy grid storagebecause they allow the energy storage capacity of the battery to bedecoupled from the power that the battery can deliver. Aqueous flowbatteries can be integrated into an electrochemical desalination systembecause they share many common desalination capital requirements such aspumps, plumbing, and cell stack designs, which can be exploited toperform both desalination and energy storage. In principle,incorporating electrical energy storage into a desalination batterywould enable further reductions in cost by facilitating load shifting onthe electrical grid, enabling electricity arbitrage, and/or enablingdeferral of investments into transmission and distributioninfrastructure. A desalination battery with a high cell potential wouldfunction as a viable energy storage device. By tapping the revenuestreams that are available to an energy storage technology, the systemcan defray desalination costs while simultaneously enabling increasedadoption of renewables.

An example of an electrochemical desalination-salination system isillustrated in FIG. 3. The described electrochemical desalinationbattery can be operated in batch mode in certain embodiments or in acontinuous mode in other embodiments. In batch mode, a volume of waterto be treated is provided (e.g., pushed) in the redox desalinationsystem. An electric potential is applied to the electrodes, and ions arecollected in the two electrodes until the salt concentration in thedesalination water chamber drops below a set limit (e.g. 5 parts perthousand (ppt) or 0.5 ppt). Then the water is removed from the system.In a continuous flow mode, water flows through the system, and the totalresidence time for a volume of water in each part of the system issufficient to achieve a desired reduction in salt concentration. Incertain embodiments, separate units can be broken up into differentstages and/or components with independently controlled electrodes toaccommodate decreasing salinity levels at each successive step during adesalination process. This can also accommodate increasing salinitylevels at each successive step during a salination process.

Turning to FIG. 3, an energy storage system 300 includes anelectrochemical desalination battery (EDB) unit 320 a. A set ofelectrical switches (S1A, S1B, S2A, S2B, S3A, S3B) is provided withinthe energy storage system 300 to provide a multi-configurationelectrical connection that connects various nodes of the energy storagesystem 300 to a power supply unit PS (which can be either AC or DCaccording to various embodiments described herein, whether or not sostated), a power (e.g. electric) grid, and/or to other nodes of theenergy storage system 300, and any combinations thereof. As used herein,an “electrical switch” refers to any device that is capable of alteringelectrical connections of a circuit. The set of electrical switches(S1A, S1B, S2A, S2B, S3A, S3B) constitutes an operational mode controldevice that controls the operational modes of the energy storage system300. As used herein, an “operational mode control device” refers to anydevice that can be employed to select an operational mode within adevice configured to operate in two or more alternative operationalmodes. The operational mode control device is configured to select,among others, between a charge, discharge, and electrodialysis modebased on presence or absence of power demand from a power grid and/oravailability of external power as provided by a power supply unit PS foroperation of the energy storage system 300.

The EDB unit 320 a includes an anode (318, 308) and a cathode (316,302), which can be embodied in various configurations. The anode (318,308) is capable of accepting and having a reversible redox reaction withthe redox shuttle dissolved in water. The cathode (316, 304) is capableof accepting and having a reversible redox reaction with the redoxshuttle dissolved in water. The anode (318, 308) includes a negativeelectrode plate 318 and an electrolyte chamber 308 for containing anelectrolyte solution (i.e., anolyte). The cathode (316, 302) includes apositive electrode plate 316 and an electrolyte chamber 302 forcontaining an electrolyte solution (i.e., catholyte). In certainembodiments, one or both of the cathode and anode includes intercalationmaterial as an optional component for battery chemistries that employintercalation. The four-chambered cell operates when both the anolyteand the catholyte have the same sign of charge (i.e., both arepositively charged or both are negatively charged).

Two electrolyte (e.g., water with dissolved salts such as sodiumchloride) chambers 304, 306 are provided between the anode (318, 308)and the cathode (316, 302), and contain solution (e.g., water) to besalinated and desalinated, respectively.

In some embodiments, the separation distance between the anode (318,308) and the cathode (316, 302) decreases along a direction of waterflow during operation. Desalination is driven by ion diffusion in theEDB unit 320 a. When the salt concentration decreases in thedesalination chamber, it takes greater effort for the ions to reach theelectrodes (i.e., the anode and the cathode), effectively increasing thecell resistance. Reducing the electrode distance at a rate that keepsthe internal resistance substantially constant leads to anenergy-efficient desalination process, as well as energy-efficientrecharging during the salination process. In certain embodiments, theseparation distance between the anode (318, 308) and the cathode (316,302) can be roughly inversely proportional to the concentration of ionsas the water in treatment passes through the water chambers 304, 306either in the charge mode or in the discharge mode. In alternativeembodiments, the separation distance between the anode (318, 308) andthe cathode (316, 302) remains substantially constant, or increases.

The negative electrode plate 318 and the positive electrode plate 316each include a solid conductive material. In a given EDB, the plates318, 316 can comprise the same conductive material or differentconductive materials. Each electrode plate 318, 316 can comprise one ormore of the following solid materials: zinc, iron, chromium, nickel,lead, titanium, copper, tin, silver, lead(IV) oxide, manganese (IV)oxide, sulfur, Prussian blue, Prussian blue derivatives, transitionmetal analogs of Prussian blue, carbon fiber, graphite, carbon felt,conductive carbon black as a solid or as an aqueous suspension, andother conductive forms of carbon. If the electrode plates 318, 316 areporous, they can occupy some portion of the anolyte chamber 308 andcatholyte chamber 302, through which the anolyte and catholyte areflowed. Alternatively, one or both of the electrode plates 318, 316 canbe constructed as a gas diffusion electrode with hydrogen gas or oxygengas as a reactant.

A first ion exchange membrane 314 is disposed between a first waterchamber 306 and the anode (318, 308). In certain embodiments, the firstion exchange membrane 314 is an anion exchange membrane (AEM) thatallows passage of anions and does not allow passage of cations, or itcan be a negative-valence-selective membrane that allows passage ofanions of lesser or greater negative charge while not allowing passageof anions of greater or lesser negative charge or positive ions. In oneembodiment, the first ion exchange membrane 314 can be a semi-permeablemembrane. An example of a material for the first ion exchange membraneis NEOSEPTA AFX by ASTOM Corporation (8 cm²).

A second ion exchange membrane 312 is disposed between the first andsecond water chambers 306, 304. In certain embodiments (e.g., when thefirst ion exchange membrane is an AEM), the second ion exchange membrane312 is a cation exchange membrane (CEM) that allows passage of cationsand does not allow passage of anions, or it can be apositive-valence-selective membrane that allows passage of cations oflesser or greater positive charge while not allowing passage of cationsof greater or lesser positive charge or negative ions. In oneembodiment, the second ion exchange membrane 312 can be a semi-permeablemembrane. An example of a material for the second ion exchange membraneis Fumasep™ FKE-50, by FuMA-Tech, GmbH, Germany (8 cm²).

A third ion exchange membrane 310 is disposed between the second waterchamber 304 and the cathode (316, 302). In certain embodiments (e.g.,when the first ion exchange membrane 314 is an AEM), the third ionexchange membrane 310 is also an AEM. In embodiments where the first ionexchange membrane 314 is a CEM, the third ion exchange membrane 310 isalso a CEM. The first and third ion exchange membranes 314, 310 are notnecessarily comprised of the same materials and/or dimensions within theEDB unit 320 a.

The EDB unit 320 a is an electrochemical cell used to treat water,including desalination and salination. Both chambers 304, 306 receivethe water to be treated. During the charging cycle, the electrolyte(e.g., salt) concentration in chamber 306 increases and the electrolyteconcentration in chamber 304 decreases. Thus, in this embodiment,chamber 306 is a salination chamber and chamber 304 is a desalinationchamber. As shown above, it is possible for these chambers to switchwhere chamber 306 is the desalination chamber and chamber 304 is thesalination chamber. However, in all embodiments, the electrolyteconcentrations in chambers 304, 306 will change in different directionsduring operation.

The water to be treated in chambers 304, 306 can include one or moreelectrolytes that may be treated. For example, the electrolyte in thechambers 304, 306 can be any combination of water-soluble ionic salts,including but not limited to, those encountered in seawater orwastewater. Example cations that can be present in the electrolyteinclude, but are not limited to, hydronium, lithium, sodium, potassium,magnesium, calcium, aluminum, zinc, and iron. Example anions that can bepresent in the electrolyte include, but are not limited to, chloride,bromide, iodide, halide oxyanions, sulfur oxyanions, phosphorousoxyanions, and nitrogen oxyanions. The system 300 is configured toremove dissolved ionic species, such as those above, from water havingan electrolyte concentration of up to the solubility limit of the ionicspecies in the desalination chamber. In certain embodiments, thatelectrolyte concentration can exceed 60 parts per thousand, and infurther embodiments, the electrolyte concentration can exceed 80 partsper thousand. The system 300 is further configured to reduce theelectrolyte concentration (e.g., salinity) of water being treated toabout 5 parts per thousand, in further embodiments, to about 2 parts perthousand, or in still further embodiments, to about 0.5 parts perthousand.

As discussed above, the EDB unit 320 a can operate as a flow battery inwhich unprocessed water is continuously supplied at inputs, andprocessed water is continuously extracted from outputs. The EDB unit 320a, or cell stack, which includes flow plates, electrodes, gaskets, andmembranes, can either have a planar geometry (similar to typical fuelcells), or the stack can comprise tubular systems that are similar inaspect ratio to reverse osmosis desalination modules.

In certain embodiments, a first water tank 330 is connected to the waterchambers 304, 306 through respective ports 332, 334. The first watertank 330 contains first-type water W1 having a first level of salinity(e.g., seawater). The pressure of the first-type water W1 can becontrolled by a first pressure controller PC1, which can apply pressureon the first-type water W1. Alternatively, water pumps (not shown) thatpush the water from respective water tanks into the EDB unit 320 a at adesired flow rate may be employed in lieu of, or in addition to,pressure controller PC1. During the charging cycle, the electrolyte(e.g., salt) concentration in chamber 306 increases and the electrolyteconcentration in chamber 304 decreases. These are output from the EDBunit 320 a to a switching unit 340 a through ports 336, 338. Hereswitches S4A, S4B direct the output water either through the system towater tank 350, or the water is discharged through respective ports 342,344. Because the four-chambered cell 320 a continuously generates adesalinated stream of water, the switching unit 340 a controls therespective salinated and desalinated stream outputs of the EDB unit 320a. For example, before the discharge cycle, the contents of chambers304, 306 are replaced by new input water from tank 330. During thedischarge cycle, the electrolyte concentration in 306 decreases and theelectrolyte concentration in 304 increases—the opposite of what occurredin the previous half-cycle. Therefore, it is necessary to alternatelycollect water passing through chambers 304, 306 and into tank 350,changing over every half-cycle.

After passing through the switching unit 340 a , the water in the secondtank 350 is a second water type W2, that is different from the firstwater type W1 (e.g., water with a different level of salinity). Forexample, when desalinated water is collected in the second tank 350, thesecond-type water may be brackish water (e.g., salinity of less thanabout 10 parts per thousand). Alternatively, the salinated water may becollected in the second tank 350 such that W2 would have a highersalinity than W1. As with the first-type water, the pressure of thesecond-type water W2 can be controlled by a second pressure controllerPC2, which can apply pressure on the second-type water W2 and/or waterpumps may be employed. The water that is discharged from either port 344or port 342 may be discharged from the system entirely or preserved inanother storage tank for further use.

The set of electrical switches also determine the operational mode ofthe EDB unit 320 a . A first set of electrical switches (S1A, S1B)controls electrical connection of the anode (318, 308) and the cathode(316, 302) of the energy storage system 300 to other electrical nodes.For clarity, the negative terminal of the EDB unit 320 a is hereinafterreferred to as the “anode” (318, 308), and the positive terminal as the“cathode” (316, 302), regardless of whether the EDB unit 320 a isoperating in charging or discharging mode. This is in view of the factthat both electrodes of a battery, including those described herein, canbe either the anode or the cathode, depending on whether the battery isbeing charged or discharged.

During operation in the charging mode, the first set of electricalswitches (S1A, S1B) can connect the anode (318, 308) to a negativeoutput voltage node of a DC power supply unit, i.e., the power supplyunit PS, and can connect the cathode (316, 302) to a positive outputvoltage node of the DC power supply unit, respectively. As used herein,“DC power supply unit” refers to a power supply unit that provides DCpower, i.e., direct current power that does not change polarity as afunction of time. Ions are released or taken up from the anode and/oranolyte (318, 308) and the cathode and/or catholyte (316, 302) tosalinate the water in the chamber 306, and desalinate the water inchamber 304. The EDB unit 320 a can be configured to have a cellpotential of at least 0.8 V, or in certain embodiments at least 1.25 V.

During operation in the discharging mode, the first set of electricalswitches (S1A, S1B) can connect the anode (318, 308) to a negativeelectrode of an electrical load, and can connect the cathode (316, 302)to a positive electrode of the electrical load, respectively. The EDBunit 320 a desalinates water in chamber 306 and salinates water inchamber 304 while releasing stored energy as output power employing theanode (318, 308) as a negative output electrode and the cathode (316,302) as a positive output electrode. As used herein, “DC output power”refers to output power provided in the form of direct current, i.e.,output power that does not change polarity as a function of time.

The electrical load can include the power grid. A second set of switches(S2A, S2B) can connect the anode (318, 308) and the cathode (316, 302)of the EDB unit 320 a to an inverter 390. The inverter 390 converts theDC output of the EDB unit 230 a to an AC power output with a matchingamplitude (i.e., the same amplitude as the amplitude of the AC voltageof the power grid) and a synchronous phase to feed into the power grid.Thus, the power released from the EDB unit 320 a during the dischargemode can be transmitted to the power grid through the first and secondsets of switches (S1A, S1B, S2A, S2B) and the inverter 390. The inverter390 can be provided as part of the energy storage system 300, or can beprovided externally on the side of the power grid. In one embodiment,the second set of switches (S2A, S2B) can be controlled by a power gridload monitor 392, which monitors the total power load on the power grid,and connects the second set of switches (S2A, S2B) with the inverter 390at, or near, the peak power demand on the power grid.

In certain embodiments, the energy storage system 300 includes furtheroptional water processing units 380. For example, tank 350 may beconnected to an additional water-processing unit via ports 352, 354. Thefurther processing units 380 may include one or more additional EDBunits, represented by cell stack 320 b . Each additional EDB unit 320 bwould include a corresponding switching unit represented by switchingunit 340 b . The one or more EDB units 320 b and switching units 340 bwould operate as discussed above for EDB unit 320 a and switching unit340 a . Other optional units may include one or more desalination units,represented by 360.

The one or more desalination units 360 can utilize a desalinationtechnique other than an electrochemical battery such as reverse osmosis,capacitive deionization, electrodialysis, and thermal techniques. In oneembodiment, the desalination unit 360 can perform a second or furtherdesalination or salination process depending upon the operation of theEDB unit 320 a . In further embodiments, even though the desalinationunits 360 are present in the system 300, they may not always be utilizedeven though the EDB unit 320 a is operational.

The desalination units 360 can have a water port (herein referred to asa third water port 362) that is connected to water having a higher totaldissolved solids (TDS) count (e.g., higher salinity), and another waterport (herein referred to as a fourth water port 364) that is connectedto water having a lower TDS count. For example, the water having thehigher TDS count can be the second-type water W2 contained within thewater tank 350 or water received from one or more optional EDB units 320b , and the water having the lower TDS count can be a third-type waterW3 contained within a third water tank 370. For example, the TDS countof the third-type water W3 can be less than about 0.5 ppt, or at a levelconsidered potable water, although a higher TDS count can also beemployed. The pressure of the third-type water W3 in the third watertank 370 can be regulated by a third pressure controller PC3, which canapply pressure on the third-type water as needed. Alternatively, waterpumps (not shown) that push the water from respective water tanks intothe one or more desalination units 360 at a desired flow rate may beemployed in lieu of, or in addition to, pressure controllers (PC2, PC3).

In certain embodiments, the direction of water flow between the secondwater tank 350 and the third water tank 370 is selected depending onwhether the one or more desalination units 360 operate in a salinationmode or in a desalination mode. The desalination units 360 canalternately operate in a desalination mode in which ions are removedfrom input water while consuming power supplied to the unit(s) 360, andin a salination mode in which ions are introduced to the input waterwhile releasing energy stored in the unit(s) 360.

During operation of one or more optional water processing units 380, atleast a fraction of the output power generated from the EDB unit 320 acan be applied to those units 380, if necessary to provide power inputfor the operation. The routing of a fraction of the output powergenerated from the EDB unit 320 a to the further water processing units380 can be effected by a third set of electrical switches (S3A, S3B),which can be connected in a parallel connection with respect to thepower grid and the second set of electrical switches (S2A, S2B). Thepower input required to operate the further units 380 is typically asmall fraction of the power stored in the EDB unit 320 a when acomparable volume of water passes through the EDB unit 320 a and theadditional units 380. Therefore, by routing a fraction of the energyreleased from the EDB unit 320 a to the optional additional units 380through the third set of switches (S3A, S3B), the one or more waterprocessing units 380 can be adequately powered, and additional power canbe released from the EDB unit 320 a to the power grid during a dischargemode.

In addition, a process control device 385 can control the operationalmodes of the various components of the energy storage system 300. Theprocess control device 385 can include a water flow control device as acomponent therein. The water flow control device controls the pressuresof the first-type water W1, the second-type water W2, and/or thethird-type water W3 through the first, second, and/or third pressurecontrol devices (PC1, PC2, PC3) or through water pumps (not shown). Thewater flow control device may be configured to induce flow of water indifferent directions, as needed during charge and discharge modes, aswell as for further salination/desalination processes.

Preliminary performance results for a cell, as described above with zincchloride/zinc metal as the anolyte and (trimethylammoniomethyl)ferrocenechloride (FcNCl) as the catholyte, are provided in FIGS. 4A-B. In FIG.4A, a high nominal cell potential (e.g., about 1.5 V) is shown thatenables high round-trip energy efficiency. FIG. 4B shows respectivesalinities for the salinate and desalinate streams as a function of thecell's state of charge (SOC). As can be seen, extensive salt removalfrom seawater solutions is possible (e.g., 35 ppt to 1.4 ppt or 96% TDSremoval). However, an 80% removal rate (e.g., 35 ppt to about 7 ppt) ismore economical. Even hypersaline brines (e.g., about 100 ppt) thatcannot be treated with reverse osmosis can be treated with the describedfour chambered cell.

As discussed above, the four-chambered electrodialytic battery producesdesalinated water during both the charge and discharge half-cycles. Thedesalinated water can achieve a salinity of at or below 0.5 ppt, butthis achievement is at the cost of lower round-trip energy efficiency.The reduction in efficiency occurs because the area specific resistance(ASR) of the battery increases sharply as the electrolyte content of thedesalinate stream drops. However, the four-chambered cell could beoperated in a third mode—as an electrodialyzer. If the anolyte, oranother redox carrier, is circulated around the catholyte and anolytechambers, a cell as described in connection with FIGS. 1A-B can operateas an electrodialyzer during times when the cell does not need to becharged or discharged. Water at an intermediate salinity (e.g., 10 ppt)produced by the device operating in an electrodialysis mode could bereintroduced to the system and further desalinated to 1.2 ppt, or evento or below 0.5 ppt (i.e., potable). The three alternative operatingmodes are described further below in connection with FIGS. 5A-C.

FIG. 5A illustrates a discharge operating mode (i.e., a firsthalf-cycle) of a four chambered electrodialytic battery. Water, such asseawater (e.g., ˜35 ppt salinity) is input 400 to the system 410. Thesystem produces electricity, which is sent to/stored at the grid 420,and a salinated water stream 430 (e.g., ˜60 ppt salinity), which isdisposed or further processed. The system 410 also produces adesalinated water stream having an intermediate salinity 440 (e.g., ˜10ppt salinity), which is stored, for example, in a holding tank 450.

FIG. 5B illustrates the second half-cycle, a charge mode of the fourchambered electrodialytic battery of FIG. 5A. Water, such as seawater(e.g., ˜35 ppt salinity) is input 400 along with electricity from thegrid 420 to the system 410. The system produces a salinated water stream430 (e.g., ˜60 ppt salinity), which is disposed or further processed.The system 410 also produces a desalinated water stream having anintermediate salinity 440 (e.g., ˜10 ppt salinity), which is stored, forexample, in a holding tank 450. The cell charge and discharge modesutilize two redox active materials as discussed above in connection withFIGS. 2A-B. A system that operates in the charge and discharge modes canalso, optionally, be operated in an electrodialyzer mode without muchadditional expense.

FIG. 5C illustrates a third electrodialysis operating mode of the fourchambered electrodialytic cell of FIGS. 5A-B that is utilized when thecell is not in charge or discharge mode. Cell operation differs however,in that in electrodialysis mode, only one redox active material isshuttled, similar to that discussed in connection with FIGS. 1A-B. Thus,while a cell configured to operate in charge and discharge modes can bereadily modified for an electrodialysis mode, the opposite is morecomplicated and cost intensive due to at least a large increase in theamount of materials.

In electrodialysis mode, water from the holding tank 450 having anintermediate salinity 440 (e.g., ˜10 ppt salinity) is input along withelectricity from the grid 420 to the system 410. The input water couldbe the desalinated output from one or both of the charge and dischargemodes described above. The system produces a salinated water stream 400(e.g., ˜35 ppt salinity) similar to the input water for thecharge/discharge modes, which is disposed and/or recycled. The system410 also produces a desalinated water stream having an even lowersalinity 460 (e.g., ˜0.5 ppt salinity). For example, the desalinatedoutput stream from the electrodialysis mode may be potable and providedto consumers as fresh water. Notably, the electrodialysis mode does notstore energy.

In each mode, feed water is continually being split into salinate anddesalinate streams. The salinities of the various feed, salinate, anddesalinate solutions can be independently varied based on flow rate,operating current density, and many other factors in variousembodiments. In further embodiments, the electrodialytic battery canproduce desalinated water at or below 0.5 ppt during charge and/ordischarge modes. The above-described modes enable the electrodialyticbattery to produce fresh water using existing hardware and mitigateand/or remove the need for a separate, secondary desalination system(e.g., a secondary reverse osmosis system).

The electrodialysis mode of FIG. 5C can be utilized in several furtherembodiments. The cell can also regenerate a concentrated aqueoussolution of a solute (i.e., a liquid desiccant), which when dissolved inwater at a sufficiently high concentration, is capable of removing waterfrom a different material that the solution is in contact with. Forexample, such a four-chambered cell in various systems may be applied tovarious processes involving dehydration and/or dessication including,but not limited to, aqueous inkjet inks, nonaqueous liquids that containdissolved water, and solid materials that contain water. Exampleembodiments may be directed to various water removal functions includingdehumidification of air, concentration of aqueous liquids, drying ofnonaqueous liquids, and dehydration of solids.

An example embodiment directed to air conditioning is described furtherin connection with FIG. 5D. Liquid desiccants have been explored forenergy efficient air conditioning of indoor air. In order to produceindoor air at a comfortable temperature and humidity (e.g., as definedby the American Society of Heating, Refrigerating and Air-ConditioningEngineers), air conditioners may need to overcool intake air in order toreduce humidity to acceptable levels. The overcooled air is thenreheated to the desired temperature using extra energy in theduplicative process.

Alternatively, a smaller amount of cooling of the intake air can becombined with a different dehumidification process in order to producedconditioned air. One approach uses liquid desiccant air conditioning toachieve the dehumidification. For example, water is absorbed, sometimethrough a membrane, into a liquid desiccant medium that has a highaffinity for water vapor. Examples of such liquid desiccants includeconcentrated aqueous solutions of ionic salts such as calcium chloride,lithium chloride, and lithium bromide. After the desiccant has absorbedwater vapor from the air, the desiccant is more diluted than prior tothe absorption and must be reconcentrated (i.e., regenerated) to at orabout the original concentration, while removing the extra water thatabsorbed from the intake air, in order to be used again.

While some approaches evaporate the excess water by heating the dilutedsolution, these approaches are inefficient due to the rate ofevaporation slowing as the salt content of the solution and relativehumidity increases. Moreover, the liquid desiccant would then need to becooled prior to beginning a new process to avoid undesirable heating ofthe air being conditioned. As discussed above in connection with FIGS.1A-B, the four-chambered cell described herein performs energy-efficientelectrodialysis in a non-thermal process. The four-chambered celloperating in an electrodialysis mode can reconcentrate a diluted liquiddesiccant solution by separating a concentrated liquid desiccant streamfrom the water absorbed from the intake air.

FIG. 5D illustrates a system incorporating a four-chambered cell in adehumidification process. A dehumidification module 470 takes input air474 (e.g., air to be cooled) and exposes the input air 474 to a liquiddesiccant having a first concentration (e.g., 40% lithium chloride) 472.The liquid desiccant 472 removes water vapor from the input air todehumidify the air. The dehumidified air is then cooled and/or furtherprocessed and output from the module 470. Upon dehumidification of theair, the liquid desiccant absorbs the water vapor from the air therebydiluting the liquid desiccant to a second concentration (e.g., 35%lithium chloride) 476. The diluted liquid desiccant 476 is then input478 into a system 410 including a four-chambered cell, as describedabove in connection with FIG. 3. For example, the dehumidificationmodule 470 would replace the first water tank 330 and input 478 wouldreplace ports 332, 334. The diluted liquid desiccant 476 would be inputto chambers 304, 306 for treatment.

The four-chambered cell is then used to reconcentrate the liquiddesiccant, which is typically a concentrated solution of inorganic salts(e.g., lithium chloride). In certain embodiments, to reduce lossesthrough water crossover from the desalinate stream to the salinatestream, the system 410 may include one or more additional, differentfour-chambered cells (e.g., represented by cell stack 320 b) in whichthe salinity is decreased in several stages to a desired level.Embodiments directed to regeneration of a liquid desiccant utilize asingle redox shuttle for energy efficiency. In some embodiments, theredox shuttle may also include some amount of dissolved inorganic saltthat acts as a supporting electrolyte while keeping all four chambers ofthe cell stack 320 b osmotically balanced. In certain embodiments, thedissolved inorganic salt is the same material as in the liquid desiccantto be regenerated. Like embodiments discussed above, the system canoperate at negligible overpotential. The single redox shuttle is alsooperated at a concentration that reduces and/or minimizes net watercrossover to or from both the salinate and desalinate streams. Anexample redox shuttle is BTMAP-Fc, discussed above, as it has a veryhigh solubility in water that can closely match, or match, theconcentrations of the desiccant streams (e.g., up to about 1.9 molar or˜10 molal), or can remain dissolved in solutions that contain a largeconcentration of dissolved inorganic salt that acts as a supportingelectrolyte while keeping all four chambers of the cell stack 320 bosmotically balanced.

The system 410 outputs a regenerated liquid desiccant stream 472 (e.g.,a salinate stream) having about the same, or the same, concentration asthe first concentration (e.g., 40% lithium chloride). This stream isthen fed back into the dehumidification module 470. Similar to FIG. 5C,the system 410 also outputs a desalinate stream 480. The desalinatestream will still include at least a small amount of dissolved saltwhich could be discharged as waste. Alternatively, the desalinate stream480 can be reused as gray water if the salt is cheap enough to discard,thereby incurring an operating cost. In further embodiments, thedesalinate stream 480 may be reconcentrated using techniques other thanelectrodialysis, such as membrane pervaporation, and fed back into anintermediate processing stage of the system 410. In HVAC embodiments,reconcentration may be performed using an evaporative method to enableadditional sensible cooling when integrated into a full HVAC system. Asdiscussed above, the dehumidification process, including regeneration ofthe liquid desiccant, is performed at a low specific energy consumptionthrough incorporation of the four-chambered desalination cell.

In summary, the above-described four (or more) chambered desalinationcell may use either one redox-active species that is circulated aroundthe anode and cathode, where it undergoes faradaic reactions at bothelectrodes, or two redox-active species that are each confined to theanode or cathode respectively. Various embodiments utilizing such a cellcan perform electrodialysis at low specific energy consumption,continuously produce desalinated and salinated water optionally coupledto electrical energy storage, whether during the charge or dischargehalf-cycle, and can enable a device that couples water desalination andenergy storage to be reversibly operated in a mode that can performelectrodialysis at low specific energy consumption. The above-describedfeatures are discusseder detail with respect to the following examples.

EXAMPLE 1

An energy-efficient redox-assisted electrodialysis system wasconstructed consistent with that shown in FIG. 1B. The anode and cathodeconsisted of three sheets of porous carbon fiber paper (SGL 39AA)pressed onto pyrosealed graphite blocks with serpentine flow channels(Entegris) and separated from the central desalinate/salinate chambersby Viton gaskets. The anion exchange membrane was Fumasep FAS-15 and thecation exchange membranes were Nafion 212.

When a solution of 0.1 M Na₃Fe(CN)₆, 0.1 M Na₄Fe(CN)₆, and 1% NaCl (50mL) was flowed past the anode/cathode and 0.6 M NaCl was in both thesalinate and desalinate chambers (50 mL each), the high-frequency ASR ofthe system was measured at 11.5 Ω·cm² and the polarization ASR was 13.7Ω·cm². FIG. 6A shows the relationship between the applied potential andthe resulting current density 602 (square points, left axis), as well asthe system polarization ASR as a function of current density 604 (roundpoints, right axis). FIG. 6B shows the relationship between the specificenergy consumption of the system as a function of current density (orNaCl transport rate). The respective experimental data points are shownas squares. The current density is directly proportional to the salttransport rate. Depending on how the cell is operated, the specificenergy consumption is lower than that of conventional electrodialysis atcurrent densities below about 80 mA/cm², and lower than that of seawaterreverse osmosis below about 20 mA/cm², though the specific energyconsumption for reverse osmosis will rise sharply with increased intakesalinity.

If necessary, the cell can produce larger volumes of desalinate at ahigher specific energy consumption. Improvements to the specific energyconsumption of the cell are expected as a result of lowered ASR. Withthis system, direct production of water at a drinkable salinity (e.g.,<0.5 ppt NaCl) is possible in a single stage from seawater (e.g., 35 pptNaCl). A constant voltage of 0.5 V was applied to the cell, causing thewater in the salinate chamber to increase in salt concentration and thewater in the desalinate chamber to decrease in salt concentration. Thesalt concentration can be decreased to at or below a thresholdconcentration of drinkable salinity. The evolution of the saltconcentration of the water in the salinate and desalinate chambers isshown in FIG. 6C as a function of charge passed and in FIG. 6D as afunction of time. At the conclusion of the experiment, the water in thedesalinate chamber had a NaCl concentration of 0.3 ppt, well below thethreshold for drinkable salinity.

To compare the redox-assisted cell with conventional electrodialysis,the specific energy consumption was modeled for a conventionalelectrodialysis process run in the same cell as described above. FIG. 7Ashows the resulting operating potential in solid lines and specificenergy consumption in dashed lines for the respective conventional andredox-assisted electrodialysis systems in the cell stack. The cell ASRis expected to be similar between conventional and redox-assistedelectrodialysis because the ASR is dominated by the membrane resistance,the ionic conductivity of the electrolytes in the intervening chambers,and the intermembrane spacing. In general, the contribution of theadditional potential to specific energy consumption for conventionalelectrodialysis may be reduced by a factor of n for every n pairs ofsalinate/desalinate chambers between each anode and cathode. But, it cannever be brought to zero. FIG. 7B provides a comparison of the relativespecific energy consumption of redox-assisted and conventionalelectrodialysis for a cell having one pair of salinate/desalinatechambers and twenty-five pairs of chambers per anode and cathode. Incertain embodiments such as the cell stack described in this example,n=1, but n=25 is typical in conventional electrodialysis.

EXAMPLE 2

In a further embodiment, BTMAP-Fc was employed in place of Fe(CN) as theredox shuttle. The use of BTMAP-Fc is expected to avoid the formation ofinsoluble solids that will be formed from the reaction of Fe(CN) withcertain metal ions. A second energy-efficient redox-assistedelectrodialysis system was constructed consistent with that shown inFIG. 1B. The anode and cathode consisted of three sheets of porouscarbon fiber paper (SGL 39AA) pressed onto pyrosealed graphite blockswith serpentine flow channels (Entegris) and separated from the centraldesalinate/salinate chambers by Viton gaskets. The anion exchangemembranes were Fumasep FAS-30 and the cation exchange membrane wasFumasep E630.

When a solution comprising 0.05 M BTMAP-Fc, 0.05 MBTMAP-Fc⁺, and 1% NaCl(50 mL) was flowed past the anode/cathode and 0.6 M NaCl was in both thesalinate and desalinate chambers (50 mL each), the high-frequency ASR ofthe system was measured at 17.9 Ω·cm², and the polarization ASR was 26.0Ω·cm². FIG. 14A shows the relationship between the applied potential andthe resulting current density 1402 (square points, left axis) as well asthe system polarization ASR as a function of current density 1404 (roundpoints, right axis). FIG. 14B shows the relationship between thespecific energy consumption of the system as a function of currentdensity (or NaCl transport rate). The respective experimental datapoints are shown as squares. The current density is directlyproportional to the salt transport rate. Depending on how the cell isoperated, the specific energy consumption is lower than that ofconventional electrodialysis at current densities below about 40 mA/cm²,and lower than that of reverse osmosis below about 10 mA/cm², though thespecific energy consumption for reverse osmosis will rise sharply withincreased intake salinity. If necessary, the cell can produce largervolumes of desalinate at a higher specific energy consumption.Improvements to the specific energy consumption of the cell are expectedas a result of lowered ASR. With this system, direct production of waterat a drinkable salinity (<0.5 ppt NaCl) is possible in a single stagefrom seawater (e.g., 35 ppt NaCl). A constant voltage of 0.5 V wasapplied to the cell, causing the water in the salinate chamber toincrease in salt concentration and the water in the desalinate chamberto decrease in salt concentration. The evolution of the saltconcentration of the water in the salinate and desalinate chambers isshown in FIG. 14C as a function of charge passed and in FIG. 14D as afunction of time. At the conclusion of the experiment, the water in thedesalinate chamber had a NaCl concentration of 0.3 ppt, well below thethreshold for drinkable salinity.

In yet another embodiment, several stages of desalination can beperformed, in which the redox-active reactant, optionally with asupporting electrolyte comprising the salt in the salinate/desalinate,is employed at an appropriate concentration relative to the saltconcentration in the salinate/desalinate in order to minimize watertransport in each stage. This reduces or minimizes energy inefficienciesarising from water transport from the salinate/desalinate chambers to orfrom the anolyte/catholyte chambers.

EXAMPLE 3

To determine what combination of cell size and current density (andtherefore water output) make the most economic sense for a given batterycapacity, a sophisticated bottom-up cost model was constructed for aZn|FcNCl battery consistent with the design of FIGS. 2A-B, with theexperimentally obtained polarization ASR value reported in the exampleabove as an input. A 1 MW, 12 MWh (delivered) system would be able toproduce 2000 m³ of brackish water per day at a salinity of about 10 pptat an optimized current density of 17.5 mA/cm². The various performancemetrics of this electrodialytic battery system are shown in FIGS. 8A-D.FIG. 8A shows the system costs and revenues for the above-described fourchambered electrodialytic battery using the described conservativepricing. Near-term (e.g., present), mid-term (e.g., 5 years), andlong-term (e.g. 10 years) refer to the price projections for the variouscomponents of the cell stack. FIG. 8B illustrates the round-trip energyefficiency performance including estimated shunt and pumping losses,while FIG. 8C shows the net system value (i.e., revenues less systemcosts) for the near-, mid-, and long-term. Also, FIG. 8D provides theprojected breakeven time, which decreases over the life of the system.As can be seen, the corresponding round-trip energy efficiency is 55%and the overall specific energy consumption for the desalination is0.196 kWh/kg NaCl (or 4.91 kWh/m³ of 10 ppt desalinate produced).

The minimum breakeven time would be 7.4 years at an internal rate ofreturn (IRR) of 13.5%, assuming a conservative price of $0.04/kWh forelectricity, $0.81/m³ for fully desalinated water, and $0.17/m³ as theadditional cost to treat the brackish water output. The Department ofEnergy cost target for energy storage ($150/kWh) is taken as a firstapproximation for the “value” of energy storage.

However, the economic case becomes even more compelling in arid parts ofthe world where energy is readily available in the form of solarinsolation or fossil fuels, and there is high demand for fresh water.Such places include Saudi Arabia and other countries around the PersianGulf In those areas, the price of fresh water is higher, around$1.35/m³, which reflects increased demand and increased difficulty indesalinating the highly saline waters of the Persian Gulf, which istypically about 45 ppt (as compared with 35 ppt for typical seawater).Also, lower prices of about $0.02/kWh for input electricity are possiblefor that region of the world. The comparable system costs, revenues, andrelevant performance metrics are shown in FIGS. 9A-D.

Near-term (e.g., present), mid-term (e.g., 5 years), and long-term (e.g.10 years) again refer to the price projections for the variouscomponents of the cell stack. FIG. 9A shows the system costs andrevenues for the above-described four chambered electrodialytic batteryusing optimistic, but attainable pricing for the Persian Gulf area. FIG.9B illustrates the round-trip energy efficiency performance includingestimated shunt and pumping losses, while FIG. 9C shows the net systemvalue (i.e., revenues less system costs) for the near-, mid-, andlong-term. Also, FIG. 9D provides the projected breakeven time, whichdecreases over the life of the system. In this scenario, recalculatedfor another 1 MW, 12 MWh system, the optimum current density would be 25mA/cm² at a round-trip energy efficiency of 42%, producing 1600 m³/dayof desalinate at 10 ppt. The specific energy consumption is projected tobe 0.293 kWh/kg NaCl removed, or 10.26 kWh/m³ of desalinate produced.The minimum breakeven time would be 4.6 years at an IRR of 21.7%.

In either scenario, a breakdown of component costs, as shown in FIG. 10,shows that the cost is dominated by the various ion-selective membranesin the cell. The IRR in both scenarios is heavily dependent on thepolarization ASR of the cell stack, which is based herein on anunoptimized experimental result. Further improvements to the ASR can bemade by refining the cell design to reduce and/or minimize theinter-membrane separation, which currently accounts for the bulk of thesystem ASR. Since the operating expenses are largely dependent upon thelifetimes of the various membranes, ligands and/or surface treatmentsmay be employed. For example, the crossover of Zn²⁺ ions across an AEMand into a seawater chamber could cause anolyte loss. Since zinc isinexpensive and easily removed downstream, some membrane crossover couldbe tolerable with periodic additions of zinc for rebalancing. However,if the measured crossover rate is unacceptably high (i.e., tooexpensive), ligands and/or surface treatments may be added to themembranes (on the catholyte side as well) to reduce crossover. Basedupon this analysis, lifetime of the system is expected to be on par withthat of conventional electrodialysis (e.g., about ten years), or longer,as the membranes are not exposed to extreme pH from water splitting orchorine evolution. The use of such electrochemical desalination flowbatteries is further described below.

FIG. 11 illustrates an example method for using a four chamberedelectrochemical desalination flow battery, as described above, in anenergy storage system. More specifically, the device operates as adesalination flow battery to store electricity and desalinate water inan alternating cycle. Since the cycles alternate, either the chargecycle or the discharge cycle could be considered “first.” In the exampleof FIG. 11, the battery is discharged to generate a saline output, adesalinate output, and store energy 1102 in a first half-cycle. In thesecond half-cycle, the electrochemical battery is charged to generate asaline output and a desalinated output 1104.

In certain embodiments, the energy used to perform the charge cycle maycome from a renewable resource such as solar or wind power. The flowbattery may be operated with either a single redox active speciescirculated around the anode and the cathode undergoing faradaicreactions at both electrodes or with two redox active species, one ateach electrode.

The method of FIG. 11 is further described below. In a dischargehalf-cycle, an electrochemical desalination battery unit is provided.The battery unit comprises at least one pair of water reservoirs, whereeach comprises an input and an output. Water (e.g., seawater having asalinity of about 35 ppt) is input to the two water reservoirs. Duringthe discharge mode, water in the first reservoir decreases in salinitybelow a threshold concentration (e.g., about 10 ppt, or in otherembodiments about 0.5 ppt), and water in the second reservoir increasesin salinity (e.g., to about 60 ppt). At least one anode comprises afirst solution of a first electrolyte material and is configured toaccept, and have, a reversible redox reaction with at least oneredox-active ion in the water of the first reservoir, and at least onecathode comprises a second solution of a second electrolyte material andis configured to accept, and have, a reversible redox reaction with atleast one redox-active ion in the water of the second reservoir. Thepair of water reservoirs is separated by a first type of exchangemembrane (e.g., AEM). A second type of exchange membrane, different fromthe first (e.g., CEM), is disposed between the at least one anode andthe first water reservoir and also disposed between the at least onecathode and the second water reservoir. During discharge, water to betreated (e.g., seawater) is transported into/through the first andsecond water reservoirs. The desalinated water from the first waterreservoir is saved, for example, in a holding tank while the salinatedwater generated in the second reservoir is removed/discharged from thesystem. Alternatively, the salinate stream could be saved, e.g., inanother holding tank, for further treatment or use. Electricity is alsogenerated and stored to a power grid. The desalinated water may thenoptionally be fed into one, or more, additional electrochemicaldesalination battery units having a similar design and/or one or moreadditional desalination systems, which use a water treatment processother than an electrochemical battery. The additional desalinationbatteries and/or desalination systems may be provided in series, in anyorder and combination. The water output from the system may be potable,having a salinity of, for example, equal to or less than 0.5 parts perthousand.

Before the other half-cycle is performed, here a charge half-cycle, thecontents of the first pair of water reservoirs is replaced with newwater to be treated (e.g., new seawater). During the charge cycle, thesalinity of the second water reservoir decreases and the salinity of thefirst water reservoir increases (the reservoirs change roles duringopposing half-cycles). Therefore, it is necessary to alternately collectwater passing through the respective water reservoirs. For example,during the charge mode, the desalinated water is collected and storedfrom the second water reservoir and the salinate stream output from thefirst reservoir is discharged. During charge, discharge, andelectrodialysis modes, the desalination battery produces desalinatedwater.

Since the desalination battery is capable of both energy storage anddesalination, and the process for desalination is coupled to energystorage, the charging, discharging, and/or idling of the battery can beperformed at different rates and durations. This variation can increase,or maximize, the amount of electrical energy stored and delivered, thesalination and/or desalination water flux, and/or the total systemrevenue. This versatility in utilization is not achievable with systemsthat are only capable of one of desalination or energy storage.

When the system above involves a single redox active species and is notcharging or discharging, it may be operated in a third, electrodialysismode to generate a saline output and a desalinated output as shown inFIG. 12. For example, the system may receive brackish water at anintermediate salinity (e.g., about 10 ppt) as an input to the cell 1202.Operating the cell as described in connection with FIGS. 1A-B providesan output of at least one salinate stream at a salinity above that ofthe input water and at least one desalinate stream at a salinity lessthan the input water such as potable water (e.g., water having TDS equalto or less than 0.5 ppt) 1204.

Further embodiments utilizing the system described above in connectionwith FIGS. 11 and 12 are directed to HVAC applications, such asdehumidification for air conditioning, and are further described in FIG.13. Before being input to the desalination system, a liquid desiccant isused to remove water (e.g., water vapor) from air 1302. The dilutedliquid desiccant is input to a system, as described above and includinga four-chambered desalination cell 1304. Treating the input liquiddesiccant (e.g., instead of seawater) generates a salinate stream ofregenerated liquid desiccant (e.g., at or near the concentration used inthe dehumidification process) and a desalinate stream 1306. The systemmay be used in this manner in various embodiments that involvenonaqueous solutions for separation into salinate and desalinatestreams.

In certain embodiments, the hybrid desalination battery may be used todesalinate seawater or brines from saline aquifers. In otherembodiments, the battery may be used to desalinate a large variety ofindustrial waste streams or geothermal brines, which can be at differentpH values and/or contain various amounts of non-aqueous solvents. Infurther embodiments, the battery can be used to selectively remove/addions to one of the water reservoirs while preserving one or moreproperties of that electrolyte such as pH, total suspended solids, andelectrical conductivity. Notably, the hybrid desalination battery may beoperated at a pH similar to that of the incoming feed (e.g., sea/saltwater), to reduce or minimize a need for pH adjustment. Also, asdiscussed above, in other embodiments, one chamber (e.g., the anolyte)may have a slightly lower pH than the other chambers.

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified in all instances by the term“about.” Accordingly, unless indicated to the contrary, the numericalparameters set forth in the foregoing specification and attached claimsare approximations that can vary depending upon the desired propertiessought to be obtained by those skilled in the art utilizing theteachings disclosed herein. The use of numerical ranges by endpointsincludes all numbers within that range (e.g. 1 to 5 includes 1, 1.5, 2,2.75, 3, 3.80, 4, and 5) and any range within that range.

The foregoing description has been presented for the purposes ofillustration and description. It is not intended to be exhaustive or tolimit the embodiments to the precise form disclosed. Many modificationsand variations are possible in light of the above teachings. Any or allfeatures of the disclosed embodiments can be applied individually or inany combination and are not meant to be limiting, but purelyillustrative. It is intended that the scope of the invention be limitednot with this detailed description, but rather, determined by the claimsappended hereto.

What is claimed is:
 1. A system for separating solvent from a saltdissolved in the solvent, comprising: an electrodialysis apparatus,comprising: a first reservoir comprising an input and an output, whereinthe salt dissolved in the solvent in the first reservoir is reducedbelow a threshold concentration during an operation mode; a secondreservoir comprising an input and an output, wherein the salt dissolvedin the solvent in the second reservoir increases in concentration duringthe operation mode; a first electrode comprising a first solution of afirst redox-active electrolyte material and configured to have areversible redox reaction with the first redox-active electrolytematerial, and accept at least one ion from the solvent in the firstreservoir; a second electrode comprising a second solution of a secondredox-active electrolyte material and configured to have a reversibleredox reaction with the second redox-active electrolyte material, anddrive at least one ion into the solvent in the second reservoir; a firsttype of ion exchange membrane disposed between the first and secondreservoirs; and a second type of ion exchange membrane, different fromthe first type, disposed between the first electrode and the firstreservoir and disposed between the second electrode and the secondreservoir.
 2. The system of claim 1, wherein the first solution and thesecond solution are the same and the first and second solutions arecirculated between the first electrode and the second electrode duringthe operation mode.
 3. The system of claim 1, wherein the solvent iswater.
 4. The system of claim 3, wherein the operation mode isconfigured to be a charge mode or a discharge mode, wherein the chargemode comprises driving ions from the first electrode and from the secondreservoir into the first reservoir forming a first salinate stream anddriving ions from the second reservoir into the second electrode forminga first desalinate stream, and the discharge mode comprises driving ionsfrom the second electrode and from the first reservoir into the secondreservoir forming a second salinate stream, and driving ions from thefirst reservoir into the first electrode forming a second desalinatestream while releasing stored energy as output power.
 5. The system ofclaim 4, further comprising a holding tank configured to store at leastone of the first and second desalinate streams, the system furtherconfigured to operate in an electrodialysis mode comprising using thesolvent from the holding tank as the input to the first and secondreservoirs and forming a third salinate stream and a third desalinatestream, the third desalinate stream having a concentration less thanthat of the first and second desalinate streams.
 6. The system of claim5, wherein the third desalinate stream comprises water having a saltconcentration of not more than about 0.5 ppt.
 7. The system of claim 1,wherein one of the first reservoir and the second reservoir generates adesalinated output during a charge mode, a discharge mode, and anelectrodialysis mode.
 8. The system of claim 1, wherein the first andsecond reservoirs comprise a first pair of reservoirs and the systemfurther comprises at least a second pair of reservoirs coupled to thefirst pair.
 9. The system of claim 1, wherein the first solution and thesecond solution are different.
 10. The system of claim 9, wherein theoperation mode is configured to be a charge mode or a discharge mode,wherein the charge mode comprises driving ions from the first electrodeand from the second reservoir into the first reservoir forming a firstsalinate stream, and driving ions from the second reservoir into thesecond electrode forming a first desalinate stream, and the dischargemode comprises driving ions from the second electrode and from the firstreservoir into the second reservoir forming a second salinate stream,and driving ions from the first reservoir into the first electrodeforming a second desalinate stream while releasing stored energy asoutput power.
 11. The system of claim 10, wherein the first and seconddesalinate streams comprise water having a salt concentration of notmore than about 0.5 ppt.
 12. The system of claim 1, further comprising aswitching unit coupled to the outputs of the first and second reservoirsand a second desalination system coupled to at least one output of theswitching unit, wherein the second desalination system uses a solventtreatment process different from the electrodialysis apparatus.
 13. Asystem, comprising: an input water source comprising water of a firstsalinity; a first electrodialytic battery unit, comprising: a firstwater reservoir coupled to the input water source and comprising anoutput, wherein water in the first water reservoir is reduced insalinity to a second salinity or increased in salinity to a thirdsalinity during an operation mode; a second water reservoir coupled tothe input water source and comprising an output, wherein salinity of thewater in the second water reservoir is changed in the opposite manner asthe water in the first water reservoir during the operation mode; afirst electrode comprising a first solution of a first redox-activeelectrolyte material and configured to have a reversible redox reactionwith the first redox-active electrolyte material, and accept at leastone ion from the water in the first water reservoir; a second electrodecomprising a second solution of a second redox-active electrolytematerial and configured to have a reversible redox reaction with thesecond redox-active electrolyte material, and drive at least one ioninto the water in the second water reservoir; a first type of exchangemembrane disposed between the first and second water reservoirs; and asecond type of exchange membrane, different from the first type,disposed between the first electrode and the first water reservoir anddisposed between the second electrode and the second water reservoir; aswitching unit comprising a first switch coupled to the output of thefirst water reservoir and a second switch coupled to the output of thesecond water reservoir.
 14. The system of claim 13, wherein theoperation mode comprises a discharge mode configured to generate energyand the system further comprises an energy storage unit to store thegenerated energy.
 15. The system of claim 13, wherein the operation modecomprises an electrodialysis mode and the switching unit is configuredto provide the water having the second salinity as input water to thefirst and second water reservoirs.
 16. The system of claim 13, furthercomprising a second desalination system coupled to the switching unit,wherein the second desalination system uses a water treatment processdifferent from the first electrodialytic battery unit.
 17. A method forseparating solvent from a salt dissolved in the solvent, comprising:providing an electrodialytic battery unit comprising: a first reservoircomprising an input and an output, wherein the salt dissolved in thesolvent in the first reservoir is reduced below a thresholdconcentration during an operation mode; a second reservoir comprising aninput and an output, wherein the salt dissolved in the solvent in thesecond reservoir increases in concentration during the operation mode; afirst electrode comprising a first solution of a first redox-activeelectrolyte material and configured to have a reversible redox reactionwith the first redox-active electrolyte material, and accept at leastone ion from the solvent in the first reservoir; a second electrodecomprising a second solution of a second redox-active electrolytematerial and configured to have a reversible redox reaction with thesecond redox-active electrolyte material, and drive at least one ioninto the solvent in the second reservoir; a first type of exchangemembrane disposed between the first and second reservoirs; and a secondtype of exchange membrane, different from the first type, disposedbetween the first electrode and the first reservoir and disposed betweenthe second electrode and the second reservoir; transporting solventhaving a first concentration of dissolved salt into the first and secondreservoirs; operating the battery unit in a first mode to generate afirst stream having a second concentration of dissolved salt that islower than the first concentration of dissolved salt and to generate asecond stream having a third concentration of dissolved salt that ishigher than the first concentration of dissolved salt.
 18. The method ofclaim 17, wherein the first mode is one of a charge mode, a dischargemode, and an electrodialysis mode.
 19. The method of claim 17, whereinoperating the battery unit in the first mode comprises generatingelectricity and further comprising storing the generated electricity.20. The method of claim 17, further comprising: emptying the first andsecond reservoirs after operating the battery unit in the first mode;transporting solvent having the first concentration of dissolved saltinto the first and second reservoirs; and operating the battery unit ina second mode, the second mode being different than the first mode, togenerate a third stream having a fourth concentration of dissolved saltthat is lower than the first concentration of dissolved salt and togenerate a fourth stream having a fifth concentration of dissolved saltthat is higher than the first concentration of dissolved salt.